mac80211: fix fast-rx encryption check

When using WEP, the default unicast key needs to be selected, instead of
the STA PTK.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20201218184718.93650-5-nbd@nbd.nameSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -4176,6 +4176,8 @@ void ieee80211_check_fast_rx(struct sta_info *sta)
 
 	rcu_read_lock();
 	key = rcu_dereference(sta->ptk[sta->ptk_idx]);
+	if (!key)
+		key = rcu_dereference(sdata->default_unicast_key);
 	if (key) {
 		switch (key->conf.cipher) {
 		case WLAN_CIPHER_SUITE_TKIP: